Plasmonic switch device and method

ABSTRACT

A plasmonic switching device and method of providing a plasmonic switching device. An example device includes a resonant cavity and an electromagnetic radiation feed arranged to couple electromagnetic radiation into the resonant cavity and at least one plasmonic mode. The resonant cavity is arranged to be switchable between: a first state in which the resonant cavity has an operational characteristic selected to allow resonance of the electromagnetic radiation at a frequency of the at least one plasmonic mode; and a second state in which the operational characteristic of the resonant cavity is adjusted to inhibit resonance of the electromagnetic radiation at a frequency of the at least one plasmonic mode.

FIELD OF THE INVENTION

Aspects and embodiments relate to: a plasmonic switching device and amethod of providing a plasmonic switching device; an electromagneticwaveguide transmission modulation device and method of providing such adevice; and a further electromagnetic waveguide transmission modulationdevice and method of providing such a device.

BACKGROUND

Photonic circuits can be equipped with photonic components which have agreater speed and can handle larger bandwidths when compared toequivalent electronic components. However, such photonic components maybe prevented from attaining particularly compact, for example,nanoscale, dimensions as a result of diffraction.

One approach to overcome the diffraction limit recognises thatutilisation of surface plasmon polaritons (SPPs) can be useful. SuchSPPs arise due to the coupling of light to free electron oscillations atan interface between a dielectric and a metal. The ability to controland manipulate light on the nanoscale via SPP modes can offer a means toconstruct compact optical components for use in applications including:data storage, information technologies and sensing. In order forplasmonic circuitry to be realized, a component which is able toefficiently operate to “switch” a signal is required. It is recognisedthat switching may occur by means of alteration of propagationcharacteristics or alteration of excitation of SPPs.

Plasmonic systems may be implemented to demonstrate activefunctionalities. Such plasmonic systems may incorporate, for example,thermo- and electro-optic media, quantum dots, and/or photochromicmolecules and such systems are achieving incremental performanceprogress in relation to switching and modulation applications. However,long switching times (>nanosecond) and/or the need for relatively strongcontrol energy (˜μJ/cm²) to observe sensible signal modulation (35% to80%) can limit the practical use of such structures in signal processingor other active opto-electronic nanodevices. It will be appreciated thatin order for active plasmonics to offer a viable technological platform,both the magnitude and speed of an employed nonlinearity, together withthe spectral/spatial tunability of that effect must be improved.

It is desired to provide an improved plasmonic device.

Optical components play a key role in industry today and can frequentlybe found in common instrumentation devices. Such devices are used toserve a broad range of applications, in areas as varied as:communications systems, health and safety, security systems, andbiometrics. Optical components provide a means to implement some keyfunctionalities and can allow the harvest, generation, conversion,processing and other manipulation of optical signals. There is constantdemand for optical devices to allow or support higher integration,portability, speed, and bandwidth, whilst performing with reduced powerconsumption.

It is desired to provide an improved optical signal manipulation device.

SUMMARY

Accordingly, a first aspect provides a plasmonic switching devicecomprising: a resonant cavity formed between surfaces, one of thesurfaces comprising a plasmonic system operable to support at least oneplasmonic mode; an electromagnetic radiation feed arranged to coupleelectromagnetic radiation into the resonant cavity and the at least oneplasmonic mode; wherein the resonant cavity is arranged to be switchablebetween: a first state in which the resonant cavity has an operationalcharacteristic selected to allow resonance of the electromagneticradiation at a frequency of the at least one plasmonic mode such thatexcitation of the at least one plasmonic mode is inhibited in theplasmonic system; and a second state in which the operationalcharacteristic of the resonant cavity is adjusted to inhibit resonanceof the electromagnetic radiation at a frequency of the at least oneplasmonic mode such that the at least one plasmonic mode is excited inthe plasmonic system.

The first aspect recognises that a cavity fed with a signal of anappropriate wavelength, or a cavity having appropriate dimensions for asignal of a given wavelength, can meet conditions for resonance of thecavity.

Furthermore, the first aspect recognises that surface plasmon polaritons(SPPs) can be excited at an interface between a metal and anotherdielectric material. SPPs can be guided along a metal-dielectricinterface. The first aspect recognises that at least one surface of aresonant cavity may be formed from appropriate materials to form aplasmonic system, and that under appropriate illumination conditions,incident photons may couple with surface plasmons at the interface andresult in excitation of a SPP which can propagate along the surface ofthe plasmonic structure.

The first aspect recognises that if a plasmonic structure forming partof a resonant cavity is illuminated by electromagnetic radiation in, forexample, the optical region of the spectrum, light may couple directlyto SPPs on the plasmonic structure by scattering from the radiationfeed. Excitation of a plasmonic mode will typically require phasematching by scattering from the radiation feed. Optical resonance in thecavity is achieved via an optical Fabry-Perot mode of the cavity. As aresult of destructive interference between the plasmonic feed and theFabry-Perot mode, the SPP intensity exhibits sharp minima at, or closeto, the resonant frequencies of the optical cavity modes. Thatphenomenon may be understood in terms of a Fano resonance, whichdescribes interaction between coupled scattering channels. This coherenteffect may arise whenever scattering may take place via two pathways,either directly into a continuum background signal, or resonantlythrough a discrete channel towards the continuum. In this case, the twochannels correspond to the sharp resonance from the Fabry-Perotresonator and the non-resonant transmission of the plasmonic feed, forexample, a opening in a surface comprising a slit.

Accordingly, provided that operational characteristics of the resonantcavity are appropriately configured and adjustable, the resonant cavitycan be arranged to be switchable between: a first state in which theresonant cavity has an operational characteristic selected to allowresonance of the electromagnetic radiation at a frequency of the atleast one plasmonic mode such that excitation of the at least oneplasmonic mode is inhibited in the plasmonic system and a second statein which the operational characteristic of the resonant cavity isadjusted to inhibit resonance of the electromagnetic radiation at afrequency of the at least one plasmonic mode such that the at least oneplasmonic mode is excited in the plasmonic system.

It will, of course, be appreciated that since an SPP is itself ahybridised electromagnetic oscillation, it has an associated wavelength.Compared with incident photons which trigger SPPs, the wavelengthassociated with the SPP can be significantly shorter.

The first aspect recognises that a plasmonic device, such as a switch,may be based on a cavity structure, the operational parameters orcharacteristic of which can be adjusted, which can, in turn, allow asignal to be controlled. For example, the switch may be determined to bein an “on” or “off” state in dependence upon detected SPPs. Inparticular, the first aspect recognises that by adjusting resonanceconditions of a cavity, a switch may be implemented.

Furthermore, owing to its design, a device in accordance with the firstaspect may be integrated with Vertical Cavity Surface Emitting Lasers(VCSELs) to provide an appropriate electromagnetic radiation source.Such an arrangement may, for example, provide a means to provide acompact integrated device for efficient SPP generation, modulation andswitching.

In one embodiment, the plasmonic system surface comprises an interfacebetween a metal and a dielectric. Accordingly, it will be appreciatedthat such an arrangement, provided materials are selected appropriately,can result in a plasmonic structure. The metal may, for example,comprise a gold, copper or silver film, foil or layer. The layer maycomprise any appropriately selected material in which the real part ofthe material permittivity can be tuned to be negative in the spectralregion of device operation. Examples of such materials include, forexample, appropriately doped semiconductors. The plasmonic structure maycomprise the entire of one surface or a proportion of one surface. Insome embodiments, both or all surfaces defining the resonant cavity maycomprise a plasmonic structure.

In one embodiment, the operational characteristic comprises at least oneeffective dimension of the cavity. Accordingly, a device in accordancewith described aspects and embodiments may function by utilising eitheroptical or plasmonic resonances, in dependence upon device dimensions.One method of operation comprises taking appropriate steps to inhibitthe coupling of electromagnetic radiation, for example, radiation in theoptical region of the spectrum, to SPPs which exist at the surface ofthe plasmonic structured surface, by employing optical Fabry Perot modeswhich can be supported by the cavity structure of the device. Such FabryPerot modes can be supported by the cavity structure when feedradiation, for example, in the optical region of the spectrum isintroduced into the cavity, provided the separation between reflectivesurfaces of the cavity, for example, metal layers, is appropriatelyselected.

In one embodiment, the dimension comprises an effective optical spacingbetween the surfaces. Accordingly, to switch between states the opticalpath inside the cavity perpendicular to the reflective surfaces, isconstructed or formed such that it can be tuned to reduce (or increase)destructive interference. Active control may be achieved by, forexample, utilising a nonlinear Kerr effect. By incorporating a nonlinearmaterial into the cavity, the refractive index of the material insidethe cavity may be optically modulated to yield the desired switching.

In one embodiment, the spacing between the surfaces comprises: a spacerlayer configured to have a variable effective refractive index. In oneembodiment, the refractive index varies upon application of a voltageacross the spacer layer utilising refractive index modulation stemmingfrom an increase in carrier concentration in the spacer layer.Accordingly, the switching of the device may be operated electrically.For example, in some embodiments, utilising refractive index modulationwhich stems from an increase in carrier concentration in, for example,Indium Tin Oxide (ITO), can be successfully used as a switchingmechanism. In order to employ such an effect, embodiments may beprovided according to which a thin multilayer structure replaces atleast one of the surface structures. In one embodiment, the thinmultilayer structure may comprise two optically transparent gold filmsseparated by a layer of each of Indium Tin Oxide (ITO) and Hafnium Oxide(HfO). When a voltage is applied, the index modulation enhancesreflection from the interface, hence effectively modifying the resonantconditions of the Fabry-Perot cavity. In one embodiment, the spacerlayer comprises a multilayer structure formed from at least one of:Indium Tin Oxide, Hafnium Oxide, Gold, Copper, or Silver. In someembodiments, the reflectivity of the one or more surfaces of the cavitymay be dynamically altered by appropriate application of appropriatelychosen voltage, light, electromagnetic radiation or other similar means.

In one embodiment, the electromagnetic radiation feed comprises: asource of electromagnetic radiation which enters the cavity via at leastone opening provided in one of the surfaces, the opening being arrangedsuch that photons scattered from the feed are coupleable to theplasmonic system. The opening may comprise a slit, a hole, or any otherappropriately dimensioned feed.

In one embodiment, the at least one opening is provided in the surfacecomprising the plasmonic system. In some embodiments, a grating or othersimilar optical source or structure can be arranged to both feed thecavity and generate SPPs when exciting the plasmonic mode. In someembodiments, a hemispherical upper reflector can be used to provide thenecessary conditions for switching.

In one embodiment, the at least one opening is configured symmetricallywithin the device such that excitation of the plasmonic mode in thedevice is symmetrical. In one embodiment, the at least one opening isarranged to have different dimensions to another of the at least oneopenings, such that excitation of the plasmonic mode in the device isasymmetrical. It will be appreciated that the lineshape of the Fanoresonance is heavily dependent on the dimensions of the resonant cavitystructure and illumination conditions. That is to say, an asymmetryparameter can be controlled by altering the effective coupling parameterbetween the continuum provided by an opening, for example, a slit, andthe discrete channel provided by the cavity, in addition to varying thephase between the two channels. In some embodiments, the phase changeassociated with the optical resonance can be harnessed to modify thephase of the SPPs launched by, for example, a slit, thus allowing for adegree of control over the direction of SPP excitation when phasematching structures, for example, double slits of different widths, areemployed.

In one embodiment, the device further comprises: a plasmonic modedetector. Accordingly, the detector may be arranged to detect whetherthe device is operating in a manner in which excitation of at least oneplasmonic mode is inhibited or supported in the plasmonic system, andthus determine whether the device is in an “off” or “on” state.

A second aspect provides a method of providing a plasmonic switchingdevice comprising: forming a resonant cavity between surfaces, one ofthe surfaces comprising a plasmonic system operable to support at leastone plasmonic mode; arranging an electromagnetic radiation feed tocouple electromagnetic radiation into the resonant cavity and the atleast one plasmonic mode; arranging the resonant cavity to be switchablebetween: a first state in which the resonant cavity has an operationalcharacteristic selected to allow resonance of the electromagneticradiation at a frequency of the at least one plasmonic mode such thatexcitation of the at least one plasmonic mode is inhibited in theplasmonic system; and a second state in which the operationalcharacteristic of the resonant cavity is adjusted to inhibit resonanceof the electromagnetic radiation at a frequency of the at least oneplasmonic mode such that the at least one plasmonic mode is excited inthe plasmonic system.

In the case of a plasmonic cavity, two surfaces may comprise a plasmonicsystem.

In one embodiment, the plasmonic system surface comprises an interfacebetween a metal and a dielectric.

In one embodiment, the operational characteristic comprises: effectivereflectivity of at least one of the surfaces of the cavity.

In one embodiment, it will be appreciated that a static electric fieldcan be used to alter or change the reflectivity of at least one of thesurfaces. That is to say, the reflectivity or mirrored nature of atleast one of the surfaces can be changed by an appropriately appliedstatic electric field.

In one embodiment, the operational characteristic comprises at least oneeffective dimension of the cavity.

In one embodiment, the dimension comprises an effective spacing betweenthe surfaces.

In one embodiment, the method comprises configuring the spacing betweenthe surfaces as a dielectric configured to have a variable effectiverefractive index.

In one embodiment, the method comprises arranging the refractive indexof the dielectric or spacer layer to vary upon application of a voltageacross the dielectric utilising refractive index modulation stemmingfrom an increase in carrier concentration in the dielectric.

In one embodiment, the method comprises: forming a surface of the cavityas a multilayer structure formed from at least one of: Indium Tin Oxide,Hafnium Oxide, Gold, Copper, or Silver.

In one embodiment, the method comprises providing an electromagneticradiation feed comprising: a source of electromagnetic radiation whichenters the cavity via at least one opening provided in one of thesurfaces, the opening being arranged such that photons scattered fromthe slit are coupleable to the plasmonic system.

In one embodiment, the method comprises: providing at least one openingin the surface comprising the plasmonic system.

In one embodiment, the method comprises: arranging the at least oneopening to have different dimensions to another of the at least oneopening, such that excitation of the plasmonic mode in the device isasymmetrical.

In one embodiment, the method further comprises: providing a plasmonicmode detector.

The general principle of operation of the first and second aspects issuch that an electromagnetic feed can be integrated into a mirror from aFabry Perot cavity, that cavity having metallic or dielectric mirrors.The feed may be non-resonant or have a very broad resonance, while theFabry Perot cavity supports sharp resonances (in the spectral domain).When the Fabry Perot cavity is driven close to resonance, it turns offthe feed through destructive interferences (this is called a Fanoresonance).

Accordingly, one further implementation of the principle of the firstaspect may provide: a switching device comprising: a resonant cavityformed between surfaces and an electromagnetic radiation feed arrangedto couple electromagnetic radiation into the resonant cavity; whereinthe resonant cavity is arranged to switch the electromagnetic feedbetween: a first state in which the resonant cavity has an operationalcharacteristic selected to allow said cavity to be is driven close toresonance such that passage of the electromagnetic feed through thecavity is inhibited; and a second state in which an operationalcharacteristic of the resonant cavity is adjusted to inhibit cavityresonance such that passage of the electromagnetic feed through thecavity is supported.

A corresponding method may also be provided,

A third aspect provides an electromagnetic waveguide transmissionmodulation device comprising: at least one hyperbolic metamaterialelement coupleable to the waveguide; the hyperbolic metamaterial elementbeing arranged to be adjustable between: a first mode in which themetamaterial element is configured to support a resonant mode matched toa propagation vector of a waveguide transmission mode supported by thewaveguide such that propagation of the waveguide transmission mode alongthe waveguide is affected; and a second mode in which the metamaterialelement is configured to inhibit support of the resonant mode matched tothe propagation vector of the waveguide transmission mode, such thatinterruption of propagation of the waveguide transmission mode along thewaveguide is prevented.

Aspects recognise that whilst high-speed optical modulators areavailable commercially with a speed ranging from 10 MHz up to 50 Ghz,those optical modulators use a Mach-Zehnder interferometer with anoptical nonlinear material (usually Lithium Niobate) in order to achievethose high speeds. Such optical modulators may be on-chip integrated,but typically have a large footprint.

Aspects also recognise that the integration of optical modulatorson-chip may be performed such that an electro-optic effect is generatedusing a very fast electrical signal in the order of tens of GHz.However, there are limitations associated with use of electro-opticeffect, since it requires considerable power for switching the opticalproperties of a material and the speed is typically limited byinterconnections carrying an electrical signal.

Aspects and embodiments recognise that there is a need to provide ameans to realise a small footprint, ultrafast, low consumption,integrated optical element, which may be used to provide a range ofoptical functionalities whilst being industrially transferable and CMOScompatible. Some embodiments may, for example, provide a device whichcan be used as an optical sensor to convert chemical information intooptical data. Other embodiments may provide a device which can be usedas a modulator, switch, or amplifier to manipulate and process photonicsignals.

Some aspects and embodiments may offer improved functionality orperformance since they may be integrally formed as part of known silicontechnologies, may be relatively simply manufactured and provide improvedoperational speed (>100 GHz for electronically drive devices), switchingtime (<ns for electronically drive devices) and/or bandwidth.

Aspects and embodiments described herein recognise that the opticalproperties of metamaterial systems based on the arrangement ofnanostructure components present an opportunity to design novel opticaldevices which may offer useful properties. Electromagnetic metamaterialscan exhibit high sensitivity to their surroundings and thus can offer ameans to create a sensitive device.

Sensing is one possible application of the proposed invention. Aspectsrecognise that whilst there are various commercial sensors available onthe marketplace to address demands in biomedical-sensing, medicaldiagnostics, or even toxic materials identification, a device inaccordance with aspects and embodiments may offer improvement. Aspectsrecognise that an optical approach to sensing generally relies onchanges in the transmittance or reflectance of the device based onenvironmental changes occurring in the proximity of the device. Such anapproach may be limited by the sensitivity level of such devices, sincethe sensitivity is constrained by weak light-matter interactions, aswith purely transparent molecules, for example. A sensitivityimprovement may be achieved by allowing for detection of nanoscaleinteractions with molecules. Efficient approaches relying on surfacewaves and other resonant effects involving surface plasmons can beimplemented. Aspects may combine such approaches whilst also allowing adevice to be spatially integrated in a waveguide, thus enabling, forexample: high throughput, multiplexing, and/or remote sensing devices.

Aspects and embodiments may provide an improved optical modulator.Aspects recognise that the basic function of an optical modulator is toencode one or more data streams on a carrier wave. The density of theinformation processed (or the channel capacity) is directly proportionalto the carrier frequency, thus making the use of optical signalsoperating at GHz-THz attractive in comparison to low frequencyelectronics typically operating in the lower GHz regime. However, mostcommercial modulators display limited performance since the drivingelectronics cannot be much faster than 100 GHz due to the naturallimitations of parasitic resistance-capacitance times. Aspects andembodiments may offer a means to overcome such issues.

The electromagnetic waveguide transmission modulation device maycomprise: at least one hyperbolic metamaterial element coupleable to thewaveguide. The hyperbolic metamaterial may comprise an anisotropicmaterial which has a negative permittivity in one direction and apositive permittivity in a transverse direction, resulting in a materialwhich displays behaviour in one direction which are similar to that ofmetallic crystals and yet displays dielectric material characteristicsin another direction. The dispersion relation of the hyperbolicmetamaterial is such that for a given frequency of electromagneticradiation and a given set of material parameters is defined by ahyperboloid.

The element may be permanently coupled or integrally formed with thewaveguide.

The hyperbolic metamaterial element being arranged to be adjustablebetween: a first mode in which the metamaterial element is configured tosupport a resonant mode matched to a propagation vector of a waveguidetransmission mode supported by the waveguide such that propagation ofthe waveguide transmission mode along the waveguide is affected. Theeffect induced may be such that propagation of the waveguidetransmission mode along the waveguide is prevented.

In a second mode, the metamaterial element is configured to inhibitsupport of the resonant mode matched to the propagation vector of thewaveguide transmission mode, such that interruption of propagation ofthe waveguide transmission mode along the waveguide is prevented. Thatis to say, reflection and other similar effects which may be induced ina transmission mode on inclusion of a different material in a waveguideare substantially non existent.

In one embodiment, the metamaterial element is configured to allowadjustment between the first and second mode, by means of modificationof optical properties of the metamaterial element. Aspects andembodiments described recognise that use of a plasmonic or hyperbolicmetamaterial element can result in a highly sensitive device. Thatsensitivity stems from the sensitivity of strong plasmonic couplingbetween individual nanostructure elements in a metamaterial to externalperturbations, for example, as a result of a physical or chemicalenvironmental change. Provision of, for example, a material sensitive toenvironmental change as an element within said metamaterial causes achange to said metamaterial as the surrounding environment changes. Thatchange, for example may result in a change to the effective permittivityof the metamaterial, which can be detected by various appropriatelychosen techniques.

According to some embodiments, it will be appreciated that monitoringthe transmission, reflection and/or absorption characteristics of thewaveguide can be used to detect or monitor the presence of a change inenvironment surrounding the waveguide.

In one embodiment, the device further comprises an adjuster and themetamaterial element is arranged to be adjusted by electro-optical,magneto-optical, acousto-optical or nonlinear optical interaction by theadjuster. Accordingly, active control of a device may be provided, theadjuster being configured to allow alteration of the optical propertiesof the metamaterial element by means of application of an appropriateelectro-optical, magneto-optical, acousto-optical or nonlinear opticalinteraction by the adjuster

In one embodiment, the metamaterial element is coupled to the waveguidein a manner which enables dynamic control over transmission, reflectionand/or absorption properties of the waveguide. Accordingly, a device inaccordance with some aspects and embodiments can be configured ordesigned to operate at a frequency close to an inflection point of atransmission versus frequency characteristic of a metamaterial element.In a configuration where a device is in the “off” position, for example,the transmission of the device can be chosen to be maximal/minimal orintermediate in dependence upon an envisaged application. If thetransmission is maximal, the propagation of light in the waveguide isnot altered by the presence of the device, since the device isconfigured such that the impedance of the waveguide and device ismatched. It will be appreciated that small changes in the opticalproperties of a device will then affect the transmission of thewaveguide.

In one embodiment, the hyperbolic metamaterial element is integrallyformed with the waveguide. Accordingly, a device can be directlyintegrated with waveguide, for example, silicon waveguide, technology.If integrated, a device in accordance with aspects and embodiments mayprovide a smaller device footprint, increase possible operatingfrequency and bandwidth and reduce power consumption when compared withalternative approaches.

In one embodiment, the metamaterial element is formed adjacent thewaveguide. In one embodiment, the metamaterial element is formed in-linewith the waveguide. In one embodiment, the metamaterial element isformed within the waveguide.

In one embodiment, the metamaterial element comprises: a support and aplurality of nanostructure elements comprising a metallic material; theplurality of nanostructure elements being configured on the support toallow said structure to act as a hyperbolic metamaterial, wherein thenanostructure elements are configured to cause a change in permittivityof the metamaterial on application of an external trigger to adjust thedevice between the first mode and the second mode. Accordingly, it willbe appreciated that various forms of metamaterial may be used toimplement a device in accordance with aspects and embodiments describedherein. In one embodiment, the metamaterial comprises an electromagneticmetamaterial. In one embodiment, the metamaterial comprises an opticalmetamaterial. In one embodiment, the adjacent nanostructure elements areconfigured on the support such that they are electromagneticallycoupled. In one embodiment, the nanostructure elements are configuredsuch that the electromagnetic field of one nanostructure elementspatially overlaps that of adjacent nanostructure elements. In oneembodiment the metallic material comprises a metal, an c negativematerial, such as an appropriately doped semiconductor or similar.

In one embodiment, the plurality of nanostructure elements compriseelongate nanostructure elements arranged such that their elongate axisis substantially parallel to the elongate axis of other nanostructureelements.

In some embodiments, the nanostructure elements are configured as anarray on the support. In some embodiments, the spacing between adjacentelements is chosen to be small in comparison to the wavelength ofradiation intended for transmission by the waveguide. The array maycomprise an irregular array. In one embodiment, the array comprises asubstantially regular array. In one embodiment, the nanostructureelements comprise a plurality of metallic nanorods. In one embodiment,the nanostructure elements are embedded within a dielectric matrix. Themetamaterial element may comprise a plurality of metallic nanorods oftunable diameter, length, and spacing distance which are aligned withrespect to one another and embedded in a dielectric matrix. Thegeometric tunability of the metamaterial provides extensive control overboth the bandwidth and the operating frequency of the device.

In one embodiment, the device comprises a plurality of metamaterialelements. Aspects and embodiments may allow ultrafast (THz) operationspeeds with tunable broadband capacity, since a device may be configuredto allow for operation with a plurality of operating frequencies. Such adevice may be configured to deal with a plurality of operatingfrequencies in a serial manner. Accordingly, a plurality of metamaterialelements may be provided, each matched to a different propagation vectorof a waveguide transmission mode.

A fourth aspect provides a method of providing a electromagneticwaveguide transmission modulation device, the method comprising:coupling at least one hyperbolic metamaterial element to the waveguide;arranging the hyperbolic metamaterial element to be adjustable between:a first mode in which the metamaterial element is configured to supporta resonant mode matched to a propagation vector of a waveguidetransmission mode supported by the waveguide such that propagation ofthe waveguide transmission mode along the waveguide is affected; and asecond mode in which the metamaterial element is configured to inhibitsupport of the resonant mode matched to the propagation vector of thewaveguide transmission mode, such that interruption of propagation ofthe waveguide transmission mode along the waveguide is prevented.

In one embodiment, the method comprises: configuring the metamaterialelement to allow adjustment between the first and second mode, by meansof modification of optical properties of the metamaterial element.

In one embodiment, the method comprises: arranging an adjuster and themetamaterial element such that the metamaterial element can be adjustedby electro-optical, magneto-optical, acousto-optical or nonlinearoptical interaction by the adjuster.

In one embodiment, the method comprises: coupling the metamaterialelement to the waveguide in a manner which enables dynamic control overtransmission, reflection and/or absorption properties of the waveguide.

In one embodiment, the method comprises: integrally forming thehyperbolic metamaterial element with the waveguide.

In one embodiment, the metamaterial element is formed adjacent thewaveguide.

In one embodiment, the metamaterial element is formed in-line with thewaveguide.

In one embodiment, the metamaterial element comprises: a support and aplurality of nanostructure elements comprising a metallic material; theplurality of nanostructure elements being configured on the support toallow said structure to act as a hyperbolic metamaterial, wherein thenanostructure elements are configured to cause a change in permittivityof the metamaterial on application of an external trigger to adjust thedevice between the first mode and the second mode.

In one embodiment, the metamaterial comprises an electromagneticmetamaterial.

In one embodiment, the metamaterial comprises an optical metamaterial.

In one embodiment, the method comprises: configuring the adjacentnanostructure elements on the support such that they areelectromagnetically coupled.

In one embodiment, the method comprises: configuring the nanostructureelements such that the electromagnetic field of one nanostructureelement spatially overlaps that of adjacent nanostructure elements.

In one embodiment, the method comprises: configuring the plurality ofnanostructure elements as an array on the support.

In one embodiment, the array comprises a substantially regular array.

In one embodiment, the nanostructure elements comprise a plurality ofmetallic nanorods.

In one embodiment, the method comprises: embedding the nanostructureelements within a dielectric matrix.

In one embodiment, the method comprises: providing a plurality ofmetamaterial elements.

A fifth aspect provides an electromagnetic waveguide transmissionmodulation device comprising: a pair of metamaterial elements arrangedin-line within the waveguide; the metamaterial elements being arrangedto be adjustable between: a first state in which the metamaterialelements operate as epsilon near zero (ENZ) metamaterial elements andform a resonant cavity within the waveguide having a transmissionfunction which allows electromagnetic radiation of a selected frequencypropagating along the waveguide to pass through the resonant cavitysubstantially unimpeded; and a second state in which operation of atleast one of the metamaterial elements as an ENZ metamaterial isprevented and the transmission function of the waveguide is modulated.

Aspects recognise that whilst high-speed optical modulators areavailable commercially with a speed ranging from 10 MHz up to 50 Ghz,those optical modulators use a Mach-Zehnder interferometer with anoptical nonlinear material (usually Lithium Niobate) in order to achievethose high speeds. Such optical modulators may be on-chip integrated,but typically have a large footprint, for example, a few hundred μm².

Aspects also recognise that the integration of optical modulatorson-chip may be performed such that an electro-optic effect is generatedusing a very fast electrical signal in the order of tens of GHz.However, there are limitations associated with use of the electro-opticeffect, since it requires considerable power for switching the opticalproperties of a material and the speed is typically limited byinterconnections carrying an electrical signal.

Aspects and embodiments recognise that there is a need to provide ameans to realise a small footprint, ultrafast, low consumption,integrated optical element, which may be used to provide a range ofoptical functionalities whilst being industrially transferable and CMOScompatible. Some embodiments may, for example, provide a device whichcan be used as an optical sensor to convert chemical information intooptical data. Other embodiments may provide a device which can be usedas a modulator, switch, or amplifier to manipulate and process photonicsignals.

Some aspects and embodiments may offer improved functionality orperformance since they may be integrally formed as part of known silicontechnologies, may be relatively simply manufactured and provide improvedoperational speed (>100 GHz for electronically drive devices), switchingtime (<ns for electronically drive devices) and/or bandwidth.

Aspects and embodiments described herein recognise that the opticalproperties of metamaterial systems based on the arrangement ofnanostructure components present an opportunity to design novel opticaldevices which may offer useful properties. Electromagnetic metamaterialscan exhibit high sensitivity to their surroundings and thus can offer ameans to create a sensitive device.

In scientific literature ENZ (epsilon-near zero) metamaterials aretypically defined as anisotropic materials for which the permittivitycan be described as a diagonal tensor where at least one but no morethan two (out of three) components vanish (epsilon=0). When the materialis represented by complex-valued permittivities, then both the real andimaginary parts of at least one, but no more than two (out of three)elements of the permittivity tensor vanish. Typical metamaterialscharacterised as being ENZ materials are often based on metal/dielectriccomposites and, as a result, while the real part of the permittivity ofinterest effectively vanishes at least one frequency, the imaginarycomponent does not, therefore satisfying the epsilon=0 condition onlypartially, defining such materials as “epsilon-near zero” materialsinstead of epsilon=0 materials. Within the context of this disclosure,and for practical purposes, it is believed that a device would performbest in the epsilon=0 condition but deviations from this condition inrelation to both the real and/or imaginary components of the vanishingpermittivity may be acceptable for device operation but may causedeterioration in overall performance. For the metamaterial examplearrangement described in more detail below, a device could retain a 20%modulation efficiency with an operational permittivity of the order of±8±i8.

The fifth aspect provides an electromagnetic waveguide transmissionmodulation device in which a pair of metamaterial elements is arrangedin-line within said waveguide. The device geometry may be such that twoENZ metamaterial elements are arranged in-line within a siliconwaveguide. The ENZ metamaterial elements may be substantially planar andmay be arranged to lie substantially transverse to the longitudinal axisof the waveguide. The planes of the ENZ metamaterial elements may besubstantially aligned, or parallel with respect to each other and can beembedded in a dielectric matrix. The dielectric matrix may comprise thewaveguide.

The metamaterial elements of the fifth aspect may be arranged to beadjustable between: a first state in which said metamaterial elementsoperate as ENZ metamaterial elements.

An “epsilon near zero” (ENZ) metamaterial structure can be fabricated invarious ways. A suitable metamaterial structure will typically comprisea plurality of metallic nanostructure elements arranged within adielectric. A metamaterial satisfying ENZ conditions occurs between ahyperbolic regime where the permittivity tensor comprises real elementsof opposite sign and an elliptical regime according to which the realpart of the permittivity tensor is positive. In the hyperbolic regimethe optical properties for given components of the electromagnetic fieldbehave much like they might within a metal and the material, exhibitinghigh reflectivity and large losses. In the elliptical regime a materialtypically behaves as a dielectric: exhibiting little reflectivity withlow losses. A ENZ metamaterial may be constructed to “balance” the tworegimes (elliptical and hyperbolic) to offer a material which has alarge reflectivity yet low losses. As a result, an ENZ metamaterialmirror may perform as a perfect mirror or at least the best mirrorpossible given selected composition materials.

The pair of ENZ metamaterial elements may be arranged to form a resonantcavity within said waveguide when in said first state. That resonantcavity may have a transmission function which allows electromagneticradiation of a selected frequency propagating along said waveguide topass through said resonant cavity substantially unimpeded. Accordingly,the pair of elements may be arranged to form a Fabry-Perot cavity wherethe two end-mirrors comprise thin layers of metamaterial, enclosing asection of the waveguide of determined length. The geometric tunabilityof the metamaterial provides extensive control over both the bandwidthand the operating frequency of the device.

The pair of metamaterial elements may be switchable to a second state inwhich operation of at least one of said metamaterial elements as an ENZmetamaterial is prevented and the transmission function of the waveguideis modulated. A device can be designed to work at a frequency close toan inflection point of the device transmission versus frequency plot asa function of geometry. In a configuration where the device is in the“off” position, for example, the transmission of the device can bechosen to be maximal/minimal or intermediate depending on a selecteddevice application. By arranging at least one of the metamaterialelements such that it can be switched or modulated by stimulus to nolonger operate as an ENZ “mirror” means that the resonant cavity is nolonger resonant and transmission of a waveguide including the pair ofmetamaterial elements can be modulated.

Sensing is one possible application of the proposed invention. Aspectsrecognise that whilst there are various commercial sensors available onthe marketplace to address demands in biomedical-sensing, medicaldiagnostics, toxic or explosive material identification, a device inaccordance with aspects and embodiments may offer improvement. Aspectsrecognise that an optical approach to sensing generally relies onchanges in the transmittance or reflectance of a device, or componentsof a device, based on environmental changes occurring in the proximityof the device. Such an approach may be limited by the sensitivity levelof such devices, since the sensitivity is constrained by weaklight-matter interactions, as with purely transparent molecules, forexample. A sensitivity improvement may be achieved by allowing fordetection of nanoscale interactions with molecules. Efficient approachesrelying on surface waves and other resonant effects involving surfaceplasmons can be implemented. Aspects may combine such approaches whilstalso allowing a device to be spatially integrated in a waveguide, thusenabling, for example: high throughput, multiplexing, and/or remotesensing devices. According to some embodiments, it will be appreciatedthat monitoring the transmission, reflection and/or absorptioncharacteristics of the waveguide can be used to detect or monitor thepresence of a change in environment surrounding the waveguide.

Aspects and embodiments may provide an improved optical modulator.Aspects recognise that the basic function of an optical modulator is toencode one or more data streams on a carrier wave. The density of theinformation processed (or the channel capacity) is directly proportionalto the carrier frequency, thus making the use of optical signalsoperating at GHz-THz attractive in comparison to low frequencyelectronics typically operating in the lower GHz regime. However, mostcommercial modulators display limited performance since the drivingelectronics cannot be much faster than 100 GHz due to the naturallimitations of parasitic resistance-capacitance times. Aspects andembodiments may offer a means to overcome such issues. It will beappreciated that when forming a device in accordance with arrangements,the ENZ metamaterial elements may be located within a waveguide at aspacing selected in relation to a frequency of interest. That is to say,the gap between ENZ “mirrors” can be selected to provide a resonantcavity at a frequency of interest.

The thickness of the ENZ elements provided may impact upon deviceoperation. In particular, it will be appreciated that with increasing“mirror” thickness the losses increase and therefore the value of thetotal transmission at the Fabry-Perot resonance decreases, thusincreasing insertion loss. Thus a thinner ENZ metamaterial element canbe beneficial. It will be appreciated that as the thickness of an ENZmaterial is reduced, the ability of the material to perform as an ENZmetamaterial may be compromised. Thus a balance must be struck betweenminimising losses and the ability of the device to function as intended.

Furthermore, it will be appreciated that for a waveguide of a givenheight, the quality of the mirror created by the ENZ metamaterialelements may be a function of a nanostructure chosen to form the ENZmetamaterial elements. In particular, if the nanostructured elementscomprise one or more metallic rods embedded with a dielectric, thequality of a resulting “mirror” may be a function of: rod diameter; rodseparation; and/or number of rod “layer” or rod “rows” forming theelement. Good reflectivity (exceeding 90%) can be achieved using ENZmetamaterial elements comprising a single row of rods, and where the ENZelement thickness is the thickness of a single metallic rod. For such anarrangement, the reflectivity of the ENZ element may depend upon: therod diameter and the rod separation. Rod diameters may, for example,vary between approximately 15 nm and approximately 70 nm. Rod centresmay be separated by approximately 55 nm or more.

Separation of the ENZ elements within a waveguide may be selected on thebasis of a propagation wavelength of interest. In particular, theapproximate separation distance may be given by an integer multiple ofd=λ/2n_(waveguidematerial). For a wavelength of 1.5 μm, and a Siliconwaveguide as an embedding material, the formula gives an approximate ENZelement separation of 215 nm.

Appropriate choice of construction in relation to the ENZ metamaterialelements of a device according to the first aspect can help to achieve alow energy consumption (pJ or fJ) when switching the elements between afirst and second state.

In one embodiment, the device further comprises an adjuster and at leastone metamaterial element is arranged to be adjusted by electro-optical,magneto-optical, acousto-optical or nonlinear optical interaction by theadjuster. Accordingly, active control of a device may be provided, theadjuster being configured to allow alteration of the optical propertiesof at least one metamaterial element by means of application of anappropriate electro-optical, magneto-optical, acousto-optical ornonlinear optical interaction by the adjuster

In one embodiment, the metamaterial elements are integrally formed withthe waveguide. Accordingly, a device can be directly integrated withwaveguide, for example, silicon waveguide, technology. If integrated, adevice in accordance with aspects and embodiments may provide a smallerdevice footprint, increase possible operating frequency and bandwidthand reduce power consumption when compared with alternative approaches.

In one embodiment, one or more metamaterial elements is formed in-linewith the waveguide. In one embodiment, one or more metamaterial elementsis formed within the waveguide. In one embodiment, one or moremetamaterial element is arranged to span the cross-section of thewaveguide.

In one embodiment, one or more metamaterial element comprises: aplurality of nanostructure elements within a dielectric matrix. Thenanostructure elements may comprise a metallic material. The pluralityof nanostructure elements may be configured within the dielectric matrixto allow said structure to act as an ENZ metamaterial, and thenanostructure elements are configured to cause a change in effectivereflectivity of the metamaterial on application of an external triggerto adjust the device between the first mode and the second mode.Accordingly, it will be appreciated that various forms of metamaterialmay be used to implement a device in accordance with aspects andembodiments described herein. In one embodiment, the metamaterialcomprises an electromagnetic metamaterial. In one embodiment, themetamaterial comprises an optical metamaterial. In one embodiment, theadjacent nanostructure elements are configured within the dielectricsuch that they are electromagnetically coupled. In one embodiment, thenanostructure elements are configured such that the electromagneticfield of one nanostructure element spatially overlaps that of adjacentnanostructure elements. In one embodiment the metallic materialcomprises a metal, or an c negative material, such as an appropriatelydoped semiconductor or similar.

In one embodiment, the plurality of nanostructure elements compriseelongated nanostructure elements arranged such that their elongate axisis substantially parallel to the elongate axis of other nanostructureelements.

In some embodiments, the nanostructure elements are configured as anarray or assembly within the dielectric. In some embodiments, thespacing between adjacent elements is chosen to be small in comparison tothe wavelength of radiation intended for transmission by the waveguide.The array may comprise an irregular array. In one embodiment, the arraycomprises a substantially regular array. In one embodiment, thenanostructure elements comprise a plurality of metallic nanorods. In oneembodiment, the nanostructure elements are embedded within a dielectricmatrix. The metamaterial element may comprise a plurality of metallicnanorods of tunable diameter, length, and spacing distance which arealigned with respect to one another and embedded in a dielectric matrix.The geometric tunability of the metamaterial elements can provideextensive control over the bandwidth and the operating frequency of thedevice.

In one embodiment, the device comprises a plurality of “pairs” ofmetamaterial elements. Aspects and embodiments may allow ultrafast (THz)operation speeds with tunable broadband capacity, since a device may beconfigured to allow for operation with a plurality of operatingfrequencies. Such a device may be configured to deal with a plurality ofoperating frequencies. Accordingly, a plurality of pairs of metamaterialelements may be provided, each pair matched to a different frequency andeach being individually and independently switchable between the firstand second state.

A sixth aspect provides a method of providing an electromagneticwaveguide transmission modulation device, the method comprising:arranging a pair of metamaterial elements in-line within the waveguide;arranging the metamaterial elements to be adjustable between: a firststate in which the metamaterial elements operate as ENZ metamaterialelements and form a resonant cavity within the waveguide having atransmission function which allows electromagnetic radiation of aselected frequency propagating along the waveguide to pass through theresonant cavity substantially unimpeded; and a second state in whichoperation of at least one of the metamaterial elements as an ENZmetamaterial is prevented and the transmission function of the waveguideis modulated.

In one embodiment, the method comprises: configuring the metamaterialelement to allow adjustment between the first and second mode, by meansof modification of optical properties of at least one of saidmetamaterial elements.

In one embodiment, the method comprises: providing an adjuster andarranging the metamaterial element to be adjusted by electro-optical,magneto-optical, acousto-optical or nonlinear optical interaction by theadjuster.

In one embodiment, the method comprises: integrally forming the pair ofmetamaterial elements are with the waveguide.

In one embodiment, the method comprises: forming the metamaterialelements in-line with the waveguide.

In one embodiment, at least one of metamaterial elements comprises: aplurality of nanostructure elements comprising a metallic material; theplurality of nanostructure elements being configured on the support toallow the structure to act as an ENZ metamaterial, wherein thenanostructure elements are configured to cause a change in reflectivityof the metamaterial on application of an external trigger to adjust thedevice between the first mode and the second mode.

In one embodiment, the metamaterial comprises: an electromagneticmetamaterial. In one embodiment, the method comprises: metamaterialcomprises an optical metamaterial.

In one embodiment, the method comprises: configuring the adjacentnanostructure elements such that they are electromagnetically coupled.

In one embodiment, the method comprises: configuring the nanostructureelements such that the electromagnetic field of one nanostructureelement spatially overlaps that of adjacent nanostructure elements.

In one embodiment, the method comprises: configuring the plurality ofnanostructure elements as an array.

In one embodiment, the nanostructure elements comprise: a plurality ofmetallic nanorods.

In one embodiment, the nanostructure elements are embedded within adielectric matrix.

In one embodiment, the method comprises: providing a plurality of pairsof metamaterial elements.

Although the third, fourth, fifth and sixth aspect have been describedin relation to an electromagnetic waveguide transmission modulationdevice, it will be appreciated that the principles described may beapplied to any material which can act to guide electromagnetic waves,and in particular to any material or geometry that will allow, forexample, light to escape and be collected.

Further particular and preferred aspects are set out in the accompanyingindependent and dependent claims. Features of the dependent claims maybe combined with features of the independent claims as appropriate, andin combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide afunction, it will be appreciated that this includes an apparatus featurewhich provides that function or which is adapted or configured toprovide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, withreference to the accompanying drawings, in which:

FIG. 1a illustrates schematically main features of a device according toone embodiment as a cross-section;

FIG. 1b is a schematic top view of the device shown in FIG. 1 a;

FIG. 2a illustrates schematically a device such as that illustrated inFIG. 1a when in an “on” state;

FIG. 2b illustrates schematically a device such as that illustrated inFIG. 1a when in an “off” state;

FIG. 2c illustrates one possible example of extinction ratios obtainedas a refractive index in an optical cavity of a device such as thatshown in FIG. 1a is varied;

FIG. 2d is a schematic illustration of a device according to oneembodiment, in which electrical operation can be implemented to offer anextinction ratio in the region of between 1-2 dB if 2V is applied acrossa multilayer surface as illustrated;

FIG. 3a illustrates schematically a device according to one embodiment;

FIG. 3b illustrates schematically an example of directionality for adouble slit structure such as that shown in FIG. 3a as cavity lengthvaries;

FIG. 4a illustrates schematically a multilayer system for use in adevice in accordance with one embodiment;

FIG. 4b illustrates schematically a device such as that illustrated inFIG. 4a when in an “on” state;

FIG. 4c illustrates schematically a device such as that illustrated inFIG. 4a when in an “off” state;

FIG. 4d illustrates an example of extinction ratios obtained as appliedvoltage is varied in a device such as that illustrated in FIG. 4 a;

FIG. 5a is a 3-dimensional representation of a modulator according toone embodiment;

FIG. 5b illustrates schematically the modulator of FIG. 5a when in an“off” state;

FIG. 5c illustrates schematically the modulator of FIG. 5a when in an“on” state;

FIG. 6 is a 3-dimensional representation of a modulator according to analternative embodiment;

FIG. 7 illustrates schematically main components of a device accordingto one arrangement;

FIG. 8 is a perspective view of a schematic representation of a deviceaccording to one arrangement;

FIG. 9 is a graphic representation of optical properties of a deviceaccording to one arrangement;

FIG. 10 is a graphic representation of real and imaginary parts ofepsilon z according to one arrangement; and

FIG. 11 is a graphic representation of optical properties of a deviceaccording to one arrangement.

DESCRIPTION OF THE EMBODIMENTS

According to some aspects and embodiments, a device based on a resonantcavity structure is provided. The device is arranged such that activecontrol of a plasmonic signal is permitted. That control is achieved byexploiting the presence of electromagnetic cavity modes in the cavitystructure.

FIG. 1a illustrates schematically main features of a device 10 accordingto one embodiment as a cross-section and FIG. 1b is a schematic top viewof the device shown in FIG. 1a . According to one embodiment, the devicemay generally comprise: two parallel metallic films 20 a, 20 b,separated by a spacer layer 30. In the embodiment shown, the metallicfilms comprise gold films. According to this embodiment, a slit 40 isprovided in the lower gold film 20 a, and that slit is illuminated frombeneath at normal incidence. It will be appreciated that in someembodiments, the slit need not be illuminated at normal incidence. Theupper gold film 20 b has a length W and width B and is spaced from thelower film 20 a by a distance S. It will be understood that the exampleshown is substantially 2-dimensional, but that a 3-dimensionalconfiguration may also be implemented.

FIG. 2a illustrates schematically a device such as that illustrated inFIG. 1a when in an “on” state. FIG. 2b illustrates schematically adevice such as that illustrated in Figure is when in an “off” state.FIG. 2c illustrates one possible example of extinction ratios obtainedas a refractive index in an optical cavity of a device such as thatshown in FIG. 1a is varied and FIG. 2d is a schematic illustration of adevice according to one embodiment, in which electrical operation can beimplemented to offer an extinction ratio in the region of between 1-2 dBif 2V is applied across a multilayer structure as illustrated. Furtherdetail regarding operation of a device as shown in FIGS. 2a to 2d is setout below:

Optical Cavity

A device in accordance with described aspects and embodiments mayfunction by utilising either optical or plasmonic resonances, independence upon device dimensions. One method of operation comprisestaking appropriate steps to inhibit the coupling of electromagneticradiation, for example, radiation in the optical region of the spectrum,to SPPs which exist at the surface of a device surface, by employingoptical Fabry Perot modes which can be supported by the cavity structureof the device. Such Fabry Perot modes can be supported by the cavitystructure when feed radiation in the optical region of the spectrum isintroduced into the cavity, provided the separation (S) betweenreflective surfaces of the cavity, for example, gold layers, issufficiently large (typically in the range of 200 nm to micrometres).

It will be understood by those skilled in the art that, in an embodimentof a device such as that shown in FIG. 2a , if a slit 40 is illuminatedby electromagnetic radiation 50 in the optical region of the spectrum,light may couple directly to SPPs 60 on the lower gold film surface 20 aby scattering from the slit. As a result of destructive interferencebetween two scattering amplitudes from the slit and Fabry-Perot cavity,the SPP intensity exhibits sharp minima at or close to the resonantfrequencies of the optical cavity modes. That phenomenon may beunderstood in terms of a Fano resonance, which describes interactionbetween coupled scattering channels. The effect arises wheneverscattering via two pathways interferes. In this case, the two channelscorrespond to the scattering from the slit and the scattering from theFabry-Perot cavity.

According to one implementation, and as described in relation to FIG. 2aand FIG. 2b , minima in the plasmonic signal may be used to define an‘off’ state of a switch and it will be understood that the cavitystructure of a device according to some embodiments effectively acts toinhibit SPP generation. The minima in a monitored plasmonic signal willtypically be below the level of a background signal when the device isin the “off” state.

To attain an ‘on’ state, as shown in FIG. 2a , the optical path insidethe cavity, perpendicular to the gold films, is constructed or formedsuch that it is tuned to reduce destructive interference. Active controlmay be achieved by, for example, utilising a nonlinear Kerr effect. Byincorporating a nonlinear material into the cavity, the refractive indexof the material inside the cavity may be optically modulated to changethe optical path inside the cavity and yield the desired switching. Thatarrangement is illustrated schematically in FIG. 2c and FIG. 2 d.

In some embodiments, the switching of the device may be operatedelectrically. For example, in some embodiments, utilising refractiveindex modulation which stems from an increase in carrier concentrationin a conductive material (oxide), for example, Indium Tin Oxide (ITO),can be successfully used as a switching mechanism. In order to employsuch an effect, embodiments may be provided according to which a thinmultilayer 200 replaces the upper reflector 20 b of the embodiment showngenerally in FIG. 1. In one embodiment, such as the embodiment shown inFIG. 2d , the thin multilayer 200 may comprise two optically transparentgold films separated by a layer of each of Indium Tin Oxide (ITO) andHafnium Oxide (HfO). When a voltage is applied, the index modulationenhances reflection from the interface, hence effectively modifying theresonance condition of the cavity. It will be appreciated that otherswitch triggers are possible, including, for example, mechanicalpressure.

In some embodiments (not shown) a grating or other optical source can bearranged to both feed the cavity and generate SPPs. That structure mayreplace the slit shown in the embodiment of FIG. 1. In some embodiments,a hemispherical upper reflector can be used to provide the necessaryconditions for switching. It will be appreciated that the embodimentshown in FIG. 1 is a substantially 2-dimensional device and that in suchan arrangement, a feed slit is most appropriate. In the case of a3-dimensional device, a suitable feed may comprise an opening in theform of a hole, rather than an elongate opening such as a slit.

FIG. 3a illustrates schematically a device according to one embodimentand FIG. 3b illustrates schematically an example of directionality for adouble slit structure such as that shown in FIG. 3a as cavity lengthvaries. Directionality is defined as the ratio of SPP intensity in aunique direction to the total SPP intensity; Directionality=1corresponds to SPPs excited only to the left, 0: only to the right, and0.5 equates to symmetric coupling.

In the embodiment shown in FIG. 3a , two feed slits are provided 40 aand 40 b in the lower gold film 20 a. Those two slits are asymmetricallylocated in length W of the device and have differing slit widths asshown. It will be appreciated that the lineshape of the Fano resonanceis heavily dependent on the dimensions of the resonant cavity structureand illumination conditions. That is to say, an asymmetry parameter canbe controlled by altering the effective coupling parameter between thecontinuum and the discrete channel, in addition to varying the phasebetween the two channels. In some embodiments, the phase changeassociated with the optical resonance can be harnessed to modify thephase of the SPPs launched by the slit, thus allowing for a degree ofcontrol over the direction of SPP excitation when phase matchingstructures, for example, double slits of different widths, are employed.

Plasmonic Cavity

FIG. 4a illustrates schematically a multilayer system for with a devicehaving a plasmonic cavity. As shown in FIG. 4a , a device 10 generallycomprises: two parallel metallic films 20 a, 20 b, separated by a spacer30, which in the embodiment shown comprises two spacer layers 30 a, 30b. In the embodiment shown, the metallic films comprise gold films andthe spacer layers a layer of each of Indium Tin Oxide (ITO) and HfO.According to this embodiment, a slit 40 is provided in the lower goldfilm 20 a, and that slit is illuminated from beneath at normalincidence.

FIG. 4b illustrates schematically a device such as that illustrated inFIG. 4a when in an “on” state in which a plasmonic resonance present inthe cavity, located either side of the slit, generates single interfaceSPPs outside the cavity. FIG. 4c illustrates schematically a device suchas that illustrated in FIG. 4a when in an “off” state in which lossesexperienced by the plasmonic modes in the cavity inhibit the excitationof SPPs on the adjacent gold film.

FIG. 4d illustrates an example of extinction ratios obtained as appliedvoltage is varied in a device such as that illustrated in FIG. 4 a.

It will be appreciated that in some embodiments, a mode of operation maybe implemented which reduces separation between the two gold films of acavity structure such as that shown schematically in FIG. 1, such thatthe structure is arranged to act as a plasmonic resonator. According tosuch embodiments, one possible implementation being shown in FIG. 4a ,spacing between reflective surfaces of the cavity, S, is typically inthe region of 10 to 50 nm. According to such embodiments, a device isoperable to support Fabry-Perot resonances based on plasmonic slotmodes, which are able to generate single interface SPPs external to thecavity and enhanced excitation occurs at the slot mode resonance.According to such embodiments, the plasmonic slot mode resonances areparallel to the reflective surfaces, for example, gold films in anarrangement such as that shown schematically in FIG. 1, and thus thewidth of the cavity (W) must be tuned to ensure resonance conditions areachieved. It will be appreciated that in this case, there may be no needto provide a feed. Plane wave excitation may be sufficient to provide aworkable device. Furthermore, it will be appreciated that the positionof a feed, for example, an opening in the form of a slit, or hole, maybe tuned to provide a desired device

It will be appreciated that, in a manner similar to that described abovein relation to a photonic cavity, incorporating layers of conductiveoxide and a dielectric, for example, ITO and HfO₂, in the reflectivewall structure of a cavity can facilitate switching of a signal with anapplied voltage. Such an implementation is shown in FIG. 4a . In thiscase, incorporation of appropriate spacer layers and application of anappropriate voltage can increase losses experienced by the slot modes asa result of increased electron density at the semiconductor/dielectricinterface.

The cavity structure of aspects and embodiments described herein canoffer high extinction ratios, together with reduced dimensions whencompared to similar systems. The structures of aspects and embodimentsdescribed herein can be tailored for integration with VCSELs, whichoffer an efficient platform for SPP excitation, allowing the realisationof an on-chip, plasmonic switch. It will be appreciated that aspects andembodiments described herein may be used in applications including, forexample, plasmonic switches and modulators, pressure sensors, acousticwave sensors and similar devices.

FIG. 5a is a 3-dimensional representation of a modulator according toone embodiment. According to one embodiment of a modulator, a devicegeometry such as that shown in FIG. 5a may be provided. Such anembodiment may be configured to be coupled to a waveguide. The device ofthe embodiment shown is based upon use of a metallic nanorod arraymetamaterial as coupled to a silicon (Si) waveguide. It will beappreciated that it is possible to implement arrangements which areprovided for non-silicon waveguides. The metamaterial in the embodimentshown comprises a plurality of metallic nanorods of tunable diameter,length, and spacing distance which are aligned with respect to oneanother and embedded in a dielectric matrix. The geometric tunability ofthe metamaterial provides extensive control over both the bandwidth andthe operating frequency of the device. In the embodiment illustrated,thin layers of gold and Tantalum oxide (Ta₂O₅) are introduced in thebottom of the metamaterial element.

It will be appreciated that the metamaterial of the device can beintegrated into or onto a Si-waveguide to form the device whose purposeis to enable a dynamical control over transmission, reflection and/orabsorption of an adjacent silicon waveguide.

It has been found that the transmittance of the Si waveguide as afunction of the optical properties of the device demonstrates strongtransmission modulation via modification of optical properties of theembedding matrix of the metamaterial by, for example, electro-optical,magneto-optical, acousto-optical or nonlinear optical interactions, orby using other nonlinearity of the metamaterial itself. A device inaccordance with some aspects and embodiments can be configured ordesigned to operate at a frequency close to an inflection point of thetransmission versus frequency characteristics. In a configuration wherea device is in the “off” position, for example, the transmission of thedevice can be chosen to be maximal/minimal or intermediate in dependenceupon an envisaged application. If the transmission is maximal, thepropagation of light in the waveguide is not altered by the presence ofthe device, since the device is configured such that the impedance ofthe waveguide and device is matched. It will be appreciated that smallchanges in the optical properties of a device will then affect thetransmission of the waveguide. FIG. 5b illustrates schematically themodulator of FIG. 5a when in an “off” state; and FIG. 5c illustratesschematically the modulator of FIG. 5a when in an “on” state.

When used as a sensor for example, a material or property to be sensedin the form of a gas, liquid or solid, may permeate the structure of themetamaterial such that the optical properties of the metamaterial arechanged or modified, that modification impacting transmission of thewaveguide.

In one embodiment, in which a waveguide including a metamaterial inaccordance with aspects and embodiments described, is configured tooperate as a modulator, ultrafast, for example, picosecond thermalproperties of both free and bound electron density in the metallicnanorods of a structure such as that shown in FIG. 5a can be used tomodulate the transmission/reflection/absorption of the waveguide.

Alternative ultrafast mechanisms based on optical properties of theembedding medium in interaction with the nanorods may also beimplemented. For example, in some embodiments, the embedding materialmay comprise an oxide or suitably chosen resonant or non-resonantmaterial.

It will be appreciated that a device in accordance with aspects andembodiments may be integrated into ultrafast photonic switches, and maybe silicon-photonics compatible. Furthermore, such a device may be usedto form part of an integrated bio- or chemical sensor.

FIG. 6 is a 3-dimensional representation of a modulator according to analternative embodiment. In the embodiment shown in FIG. 6, themetamaterial element is formed in-line and integrally with thewaveguide, rather than being located adjacent a wave guide as in theembodiment shown in FIG. 5.

FIG. 7 illustrates schematically the main components of a deviceaccording to one arrangement. The device geometry of the arrangementshown comprises: two ENZ metamaterial elements arranged in-line within asilicon waveguide. The ENZ metamaterial elements in the arrangementshown are integrated into a waveguide. The ENZ metamaterial elements maybe substantially planar and are arranged to lie substantially transverseto the longitudinal axis of the waveguide. The planes of the ENZmetamaterial elements may be substantially aligned, or parallel withrespect to each other and can be embedded in a dielectric matrix. Thedielectric matrix may comprise the waveguide.

It will be appreciated that an ENZ metamaterial structure can befabricated in various ways. A suitable metamaterial structure willtypically comprise a plurality of metallic nanostructure elementsarranged within a dielectric. A metamaterial satisfying ENZ conditionsoccurs in anisotropic media between hyperbolic and elliptic regimes. Inthe hyperbolic regime a material typically exhibits high reflectivity atan interface with another material but has large losses. In the ellipticregime a material typically exhibits low reflectivity at an interfacewith another material with no losses. An ENZ metamaterial may beconstructed to “balance” the two properties to offer a material whichhas a large reflectivity at an interface with another material yet lowlosses. As a result, an ENZ metamaterial mirror may perform as a perfectmirror or at least the best mirror possible given selected compositionmaterials.

It will be appreciated that a modulation device in accordance with thearrangement shown in FIG. 7 represents a Fabry-Perot cavity where thetwo end-mirrors comprise thin layers of metamaterial, enclosing asection of the waveguide of determined length. The geometric tunabilityof the metamaterial (which can be a multilayer) provides extensivecontrol over both the bandwidth and the operating frequency of thedevice. ENZ materials provide extensive spectral tunability, lowmaterial losses (the mirror formed is an effective medium with low lossymaterial content), and strong ultrafast response, due to thenanostructured composition of the mirrors.

The metamaterial elements enable ultrafast dynamic control over thetransmission, reflection and/or absorption properties of the waveguide.In particular, the transmittance of the Si waveguide as a function ofthe optical properties of the device demonstrates strong transmissionmodulation via the modification of the optical properties of themetamaterial by electro-optical, magneto-optical, acousto-optical ornonlinear optical interactions.

A device can be designed to work at a frequency close to an inflectionpoint of the device transmission versus frequency plot. In aconfiguration where the device is in the “off” position, for example,the transmission of the device can be chosen to be maximal/minimal orintermediate depending on a selected device application. If thetransmission is maximal, the propagation of a selected frequency ofelectromagnetic radiation in the waveguide may be such that is notaltered by the presence of the device. The impedance of the device ismatched to the impedance of the waveguide. Small changes in the opticalproperties of the device affect the transmission of the waveguide. Whenused as a sensor, for example, a material to be sensed (gas, liquid,solid) may permeate or be in the vicinity of the metamaterial, modifyits optical properties and affect the transmission of the waveguide. Ina simple configuration as a modulator, the ultrafast (femtosecond)thermal properties of both the free and bound electron density in, forexample, the metallic nanostructures provided in the ENZ metamaterialcan be used to modulate the transmission/reflection/absorption of thewaveguide. In some arrangements alternative ultrafast mechanisms basedon the optical properties of the embedding medium in interaction withthe metallic nanostructures forming the metamaterial may also beutilised.

It will be appreciated that when forming a device in accordance witharrangements, the ENZ metamaterial elements may be located within awaveguide at a spacing selected in relation to a frequency of interest.That is to say, the gap between ENZ “mirrors” can be selected to providea resonant cavity at a frequency of interest.

Similarly, it will be appreciated that in order to perform acceptably,the thickness of the ENZ elements provided may impact upon deviceoperation. In particular, it will be appreciated that with increasing“mirror” thickness the losses increase and therefore the value of thetotal transmission at the Fabry-Perot resonance decreases, thusincreasing insertion loss. Thus a thinner ENZ metamaterial element canbe beneficial. It will be appreciated that as the thickness of an ENZmaterial is reduced, the ability of the material to perform as an ENZmetamaterial may be compromised. Thus a balance must be struck betweenminimising losses and the ability of the device to function as intended.

In the general arrangement shown schematically in FIG. 7, bothmetamaterial elements are configurable to operate in an ENZ condition,which means the metamaterial elements can be switched between operatingas a mirror being arranged to have a transmission close to 1. The closerthe transmission is to 1 in the Fabry-Perot cavity the lower theinsertion loss and the better the modulator may operate.

FIG. 8 is a 3-dimensional representation of a modulator according to oneembodiment. In the arrangement shown in FIG. 2 the ENZ “mirrors” areformed from metallic rods in a dielectric material. The ENZ metamaterialelements in the arrangement shown each comprise a metallic nanorodarray. The ENZ metamaterial elements are formed from a plurality ofmetallic nanorods. The dimensions and arrangement of the nanorods and ofthe metamaterial elements within the waveguide may be chosen to performaccording to a proposed application of a device. Such tunability of adevice to an envisaged application can be achieved in relation to theENZ metamaterial elements since the metallic nanorods forming the ENZmetamaterial elements may have, for example: a tunable diameter, lengthand/or spacing distance.

As described in relation to the general arrangement shown in FIG. 7, aninteresting behaviour is exhibited by waveguide structures comprisingconsecutive nanorod slabs. At the ENZ condition, the metamaterialelement slab becomes highly reflective (as n=0 reflection coefficient=1)which leads to the creation of standing waves within the device. Such anarrangement can allow high transmission through the device in the samemanner as a Fabry-Perot resonator. In reality, the ENZ metamaterialcomprises a metallic nanorod structure, which can have large losses. Thestanding waves cannot be supported unless the losses are sufficientlysmall. One way to reduce losses is to reduce the number of rods needed(thus reducing the amount of metal) to create an ENZ slab.

Consider, for example, the structure shown schematically in FIG. 8. Fora certain value of the gap, the transmission of the structure is 1 for agiven frequency due to the creation of a standing wave within the device(Fabry-Perot resonance). The thickness of the ENZ metamaterial elements(slabs) can lead to significant losses. Losses can be minimized byreducing the length (thickness) of the ENZ material, although care mustbe taken since typically an ENZ structure will be fabricated inaccordance with conditions derived from an effective medium theory of aninfinite slab and therefore the “length” of the slab should not bereduced further than a single unit of the effective medium (in this caseone nanorod diameter).

The structure shown schematically in FIG. 8 comprises two ENZmetamaterial elements. The ENZ elements comprise a plurality of goldnanostructure rods embedded in a dielectric medium.

FIGS. 9 to 11 illustrate graphically a mathematical analysis of thetransmission of a structure such as that shown in FIG. 8. The analysisassumes an effective medium theory for a metamaterial slab having alength of one nanorod diameter (50 nm) the model allows both the roddiameter and gap between ENZ elements to be changed to scan theeffective permittivity of the medium through the ENZ condition and toobtain the condition for maximum transmission. A simple transfer matrixmethod (TMM) analysis is implemented to calculate the transmission ofthe device. In relation to FIG. 9 the surface plot represents thetransmittance of the modulator as a function of inter-rod distance androd diameter. The 2-D plots illustrate the real and imaginary parts ofthe z-component of the permittivity tensor as a function of roddiameter. The x,y components are positive and not dispersive. ENZconditions are achieved for a rod diameter of around 35 nm.

The structure shown in FIG. 8 is simulated fully in 3D, accounting forthe nanostructured geometry of the metamaterial using COMSOL, and itstransmission is calculated for a first TM-like mode (transversemagnetic) and plotted against rod diameter and gap. In the simulatedstructure a glass substrate is assumed, together with a mode frequencycorresponding to a free space wavelength of 1.5 um. The waveguidesimulated is 300 nm wide and 340 nm high.

FIG. 9 illustrates the transmission of one device against both rod gapand diameter using TMM. In the mathematical example shown, amathematical model comprising nanorods embedded within silicon havingn=3.48 is used to allow for comparison with real fabricated devices. Inother words, the mathematical model can be compared with empiricalresults from fabricated prototypes formed in accordance with thoseparameters. The real and imaginary part of eps_z are also plotted inFIG. 9 for ease of reference.

FIG. 10 illustrates the transmission against both gap and diameter forthe case of the 3D simulation. The gap in the finite element 3D model isdifferent to the gap in the TMM in the sense that while in the TMM theminimum length of the slab is related to the size of a unit cell (givenby the period of a nanorod array), whereas in the model of FIG. 10 thenanorods can be as close as desired, subject to the diameter of thenanorod, and analysis can be performed.

For a rod diameter of 35 nm and a spacing of 80 nm which corresponds tothe increased transmission in FEM simulations, the wavelength behaviourwas calculated and is shown in FIG. 11. The blue transmission profilecorresponds to a full 3D finite element (FEM) calculation of themodulator. The TMM plot refers to an equivalent effective medium theorycalculation.

Simulations reveal that for a single cell of effective medium the lossesare higher than for a single real cell (comprising a single rod inside awaveguide). This can be understood since in the example arrangementsconsidered the single cell is geometrically closer to a gold layer offew nanometres (related to the diameter of the rod) than to an array ofrods with an effective loss. Such an analysis can explain the increasedtransmission in the case of FEM.

Calculations were also done to simulate use of a AAO instead of siliconas an embedding medium. Although stronger resonances are seen insimulations of such an arrangement, the simulation also indicates thatmismatch between a propagating mode in silicon and in AAO, can lead to adrop in transmission through a real nanostructured component. Awaveguide made of AAO is likely to give better results, although an AAOwaveguide typically requires a larger height of the waveguide and longerrods (in the order of 500 or 600 nm). The size of the device can be aslarge as 180 nm from the FEM simulation, and simulations indicate thatwith a drop in the total transmission of 0.17, the device has relativelysmall insertion losses and integrability. Furthermore, if the device isused as a modulator: in the ON state it has low transmission and in theOFF state it has large transmission. Therefore this device may beparticularly energy efficient when integrated into an optical circuitcompared to alternative modulator arrangements.

For the case investigated using gold nanorods, it has been determinedthat the maximum transmission at the Fabry-Perot resonance condition is0.86 dropping down to 0.5 under optical pumping. It will be appreciatedthat other materials may provide an improved performance.

Use of rods in the illustrated example gives a tuned ENZ condition atthe selected working wavelength (1.5 um). In the case of the illustratedsimulated modulator the anisotropic nature of the rods have the resultthat the switchable ENZ behaviour will only work for TM polarized modes(electric field along the longitudinal axis of the rod) however for TEpolarized modes (electric field perpendicular to the longitudinal axisof the rod) the permittivity is not zero but close to that of thesilicon waveguide and therefore it will be transparent at thispolarization. Such an effect can, in some arrangements, then be used forpolarization modulation.

General Structure

It will be appreciated that it is possible to implement arrangementswhich are provided for non-silicon waveguides. The metamaterial in theembodiment shown comprises a plurality of metallic nanorods of tunablediameter, length, and spacing distance which are aligned with respect toone another and embedded in a dielectric matrix. The geometrictunability of the metamaterial provides extensive control over both thebandwidth and the operating frequency of the device.

It will be appreciated that a device in accordance with aspects andembodiments may be integrated into ultrafast photonic switches, and maybe silicon-photonics compatible. Furthermore, such a device may be usedto form part of an integrated bio- or chemical sensor.

Although illustrative embodiments of the invention have been disclosedin detail herein, with reference to the accompanying drawings, it isunderstood that the invention is not limited to the precise embodimentand that various changes and modifications can be effected therein byone skilled in the art without departing from the scope of the inventionas defined by the appended claims and their equivalents.

1. A plasmonic switching device comprising: a resonant cavity formedbetween surfaces, one of said surfaces comprising a plasmonic systemoperable to support at least one plasmonic mode; an electromagneticradiation feed arranged to couple electromagnetic radiation into saidresonant cavity and said at least one plasmonic mode; wherein saidresonant cavity is arranged to be switchable between: a first state inwhich said resonant cavity has an operational characteristic selected toallow resonance of said electromagnetic radiation at a frequency of saidat least one plasmonic mode such that excitation of said at least oneplasmonic mode is inhibited in said plasmonic system; and a second statein which said operational characteristic of said resonant cavity isadjusted to inhibit resonance of said electromagnetic radiation at afrequency of said at least one plasmonic mode such that said at leastone plasmonic mode is excited in said plasmonic system.
 2. The plasmonicswitching device according to claim 1, wherein said plasmonic systemsurface comprises an interface between a metal and a dielectric.
 3. Theplasmonic switching device according to claim 2, wherein saidoperational characteristic comprises: reflectivity of at least one ofsaid surfaces of said cavity.
 4. The plasmonic switching deviceaccording to claim 3, wherein one of said surfaces is configured to havea variable effective refractive index and the resonance condition ofsaid resonant cavity is changed as said effective reflectivity of saidsurface is varied.
 5. The plasmonic switching device according to claim4, wherein said operational characteristic comprises: at least oneeffective dimension of said cavity.
 6. The plasmonic switching deviceaccording to claim 5, wherein said dimension comprises an effectivespacing between said surfaces.
 7. The plasmonic switching deviceaccording to claim 6, wherein said spacing between said surfacescomprises a dielectric configured to have a variable effectiverefractive index.
 8. The plasmonic switching device according to claim7, wherein said refractive index varies upon application of a voltage oroptical field across said dielectric enabling refractive indexmodulation stemming from an increase in carrier concentration in saiddielectric.
 9. The plasmonic switching device according to claim 8,wherein said dielectric comprises a multilayer structure formed from atleast one of: Indium Tin Oxide, Hafnium Oxide, Gold, Copper, or Silver.10. The plasmonic switching device according to claim 9, wherein saidelectromagnetic radiation feed comprises: a source of electromagneticradiation which enters said cavity via at least one opening provided inone of said surfaces, said opening being arranged such that photonsscattered from said opening are coupleable to plasmons at said plasmonicsystem.
 11. The plasmonic switching device according to claim 10,wherein said at least one opening is provided in said surface comprisingsaid plasmonic system.
 12. The plasmonic switching device according toclaim 10, wherein said at least one opening is configured symmetricallywithin said device such that excitation of said plasmonic mode in saiddevice is symmetrical.
 13. The plasmonic switching device according toclaim 10, wherein said at least one opening is arranged to havedifferent dimensions to another of said at least one opening, such thatexcitation of said plasmonic mode in said device is asymmetrical. 14.The plasmonic switching device according to claim 10, wherein saiddevice further comprises: a plasmonic mode detector.
 15. A method ofproviding a plasmonic switching device comprising: forming a resonantcavity between surfaces, one of said surfaces comprising a plasmonicsystem operable to support at least one plasmonic mode; arranging anelectromagnetic radiation feed to couple electromagnetic radiation intosaid resonant cavity and said at least one plasmonic mode; arrangingsaid resonant cavity to be switchable between: a first state in whichsaid resonant cavity has an operational characteristic selected to allowresonance of said electromagnetic radiation at a frequency of said atleast one plasmonic mode such that excitation of said at least oneplasmonic mode is inhibited in said plasmonic system; and a second statein which said operational characteristic of said resonant cavity isadjusted to inhibit resonance of said electromagnetic radiation at afrequency of said at least one plasmonic mode such that said at leastone plasmonic mode is excited in said plasmonic system. 16-48.(canceled)
 49. A switching device comprising: a resonant cavity formedbetween surfaces; and an electromagnetic radiation feed arranged tocouple electromagnetic radiation into the resonant cavity; wherein theresonant cavity is arranged to switch the electromagnetic feed between:a first state in which the resonant cavity has an operationalcharacteristic selected to allow said cavity to be is driven close toresonance such that passage of the electromagnetic feed through thecavity is inhibited; and a second state in which an operationalcharacteristic of the resonant cavity is adjusted to inhibit cavityresonance such that passage of the electromagnetic feed through thecavity is supported.
 50. The method according to claim 15, wherein oneof said surfaces comprises a variable effective refractive index, andwherein a resonance condition of said resonant cavity is changed as saideffective reflectivity of said surface is varied.
 51. The methodaccording to claim 15, further comprising forming an effective spacingbetween said surfaces of the resonant cavity.
 52. The method accordingto claim 51, wherein the effective spacing between said surfaces of theresonant cavity comprises a dielectric having a variable effectiverefractive index.
 53. The method according to claim 52, wherein saidrefractive index varies upon application of a voltage or optical fieldacross said dielectric.